In accordance with the rapid spread of portable electronic equipment, specifications required for batteries used in such equipment have become strict year after year. Especially, batteries with a compact and slim size, high-capacity, excellent cycle characteristics, and stable performance are required. In addition, in the field of secondary batteries, nickel hydrogen batteries and lithium nonaqueous electrolyte secondary batteries with higher energy density than other batteries have been drawing attention, and the market share of these secondary batteries in the secondary battery market has shown a large increase.
In equipment using this kind of secondary batteries, since a space for containing the battery is often prismatic (flat box shape), sealed secondary batteries manufactured by sealing prismatic battery outer cans containing electric power generating elements are often used. An example of the prismatic sealed secondary batteries will be described using FIGS. 1 to 3.
FIG. 1 is a perspective view showing a longitudinal section of a lithium nonaqueous electrolyte secondary battery as a prismatic sealed battery manufactured in the related art. FIG. 2 is a plan view showing a state where a sealing plate is fitted with a battery outer can. FIG. 3 is a schematic sectional view showing a state of laser welding of the battery outer can and the sealing plate.
A sealed secondary battery 10 is manufactured as follows: a positive electrode plate 12 and a negative electrode plate 11 with a separator 13 interposed therebetween are wound to form a flat spiral electrode 14; the electrode 14 is contained in a prismatic battery outer can 15; and the prismatic battery outer can 15 is sealed with a sealing plate 16. The flat spiral electrode 14 is wound so that the positive electrode plate 12 is positioned on the outermost periphery to be exposed, and the exposed positive electrode plate 12 on the outermost periphery contacts directly and is electrically connected to the inner surface of the prismatic battery outer can 15 also serving as a positive electrode terminal. Furthermore, the negative electrode plate 11 is electrically connected to a negative electrode terminal 18 which is formed at the center of a sealing plate 16 and is attached to the sealing plate through an insulator 17, through a collector 19. Then, since the prismatic battery outer can 15 is electrically connected to the positive electrode plate 12, in order to prevent short circuit between the negative electrode plate 11 and the prismatic battery outer can 15, an insulating spacer 20 is inserted between the upper end of the flat spiral electrode 14 and the sealing plate 16. Thus, the negative electrode plate 11 and the prismatic battery outer can 15 are in an electrically insulated state.
The prismatic nonaqueous electrolyte secondary battery is manufactured as follows: the flat spiral electrode 14 is inserted in the prismatic battery outer can 15; the sealing plate 16 is laser-welded to the opening of the prismatic battery outer can 15; a nonaqueous electrolyte is poured from an electrolyte injecting hole 21; and the electrolyte injecting hole 21 is sealed up. Such a method of fixing the sealing plate 16 to the prismatic outer can by laser welding is widely used due to the advantage that the opening of the battery outer can 15 can be sealed reliably without deterioration of volumetric efficiency (see JP-A-2000-268781, JP-A-2005-183360, and JP-A-2006-19089).
As for the laser welding between the battery outer can 15 and the sealing plate 16, as shown in FIG. 3, the battery outer can 15 and the sealing plate 16 are fitted so that both of the top faces are made horizontal, the integrated assembly is irradiated with a laser beam from above in a vertical direction, and the battery outer can and the sealing plate are melted and welded. As forming materials for the above-mentioned battery outer can 15 and the sealing plate 16, aluminum or an aluminum alloy with good thermal conductivity is used for low cost and the purpose of weight reduction.
When aluminum or the aluminum alloy with good thermal conductivity is used as a forming material for the battery outer can 15 and the sealing plate 16, if sufficient welded part between the battery outer can 15 and the sealing plate 16 is not formed, the joint strength can not be enhanced, and may lead to leakage of an electrolyte at the time of drop impact. One reason why such sufficient welded part cannot be formed is that melting patterns between the battery outer can 15 and the sealing plate 16 are different from each other because of the difference of the wall thicknesses between the battery outer can 15 and the sealing plate 16 on the fitted face or the difference of the materials thereof. Thus, stable sufficient welded part cannot be obtained.
The inventors performed intensive studies in order to solve the problem which occurs when such prismatic battery outer cans 15 and the sealing plate 16 are welded by a high energy beam such as a laser beam. As a result, they found that, considering the focusing diameter of the laser beam, by designing wall thicknesses of both the battery outer can 15 and the sealing plate 16 so as to melt their entire top faces, and furthermore, in order not to deteriorate the strength of the base metal of the sealing plate 16, by making the flange bottom thick enough and making the welded part of the prismatic battery outer can 15 and the sealing plate 16 to weld sufficiently, the strength of the welded part is improved and the base metal strength of the periphery can be maintained, whereby a sealed battery with high fall resistance reliability can be obtained.